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<jats:title>Abstract</jats:title> <jats:p>The aim of this study is to generate maps of the hard X-ray emission produced by energetic electrons in a solar flare and compare them with observations. The ultimate goal is to test the viability of the combined MHD/test-particle approach for data-driven modeling of active events in the solar corona and their impact on the heliosphere. Based on an MHD model of X-class solar flare observed on 2017 September 8, we calculate trajectories of a large number of electrons and protons using the relativistic guiding-center approach. Using the obtained particle trajectories, we deduce the spatial and energy distributions of energetic electrons and protons, and calculate bremsstrahlung hard X-ray emission using the “thin-target” approximation. Our approach predicts some key characteristics of energetic particles in the considered flare, including the size and location of the acceleration region, energetic particle trajectories and energy spectra. Most importantly, the hard X-ray bremsstrahlung intensity maps predicted by the model are in good agreement with those observed by RHESSI. Furthermore, the locations of proton and electron precipitation appear to be close to the sources of helioseismic response detected in this flare. Therefore, the adopted approach can be used for observationally driven modeling of individual solar flares, including manifestations of energetic particles in the corona, as well as the inner heliosphere.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Blazars are relativistic magnetized plasma outflows from supermassive black holes that point very close to our line of sight. Their emission is nonthermal-dominated and highly variable across the entire electromagnetic spectrum. Relativistic magnetic reconnection has been proposed as the driver of particle acceleration during blazar flares. While recent particle-in-cell (PIC) simulations have self-consistently studied the evolution of magnetic reconnection and particle acceleration therein, the resulting radiation signatures have not been systematically explored. In particular, the polarization signatures, which directly reflect the characteristic strongly dynamical evolution of magnetic field during reconnection, have not been carefully investigated. In this paper, we present a systematic study of radiation and polarization signatures arising from magnetic reconnection in blazars, based on combined PIC and polarized radiation transfer simulations with various physical parameters. We identify a harder-when-brighter trend in the spectral evolution. Moreover, higher-frequency bands (ultraviolet to X-ray) tend to flare earlier than lower-frequency bands (infrared to optical) in the synchrotron spectral component. Most importantly, polarization signatures appear more variable with higher frequencies. We find that the variation in temporal polarization depends strongly on the guide field strength. Specifically, reconnection with a significant guide field component leads to a very high polarization degree that contradicts typical blazar observations, while large polarization angle rotations are unique signatures of magnetic reconnection between nearly antiparallel magnetic field lines. These rotations are at least 90° and can extend to &gt;180°, and they may be in either direction. These results imply that blazars that have shown large polarization angle rotations intrinsically have more nearly antiparallel magnetic field morphology.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a test of the statistical method introduced by Bernard F. Shutz in 1986 using only gravitational waves to infer the Hubble constant (H<jats:sub>0</jats:sub>) from GW190814, the first high-probability neutron-star–black hole (NS–BH) merger candidate detected by the Laser Interferometer gravitational-wave Observatory (LIGO) and the Virgo interferometer. We apply a baseline test of this method to the binary neutron star (BNS) merger GW170817 and find <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{H}}}_{0}={70}_{-18.0}^{+35.0}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb5f9ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup> (maximum a posteriori and 68.3% highest density posterior interval) for a galaxy <jats:italic>B</jats:italic>-band luminosity threshold of <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{B}\geqslant 0.001\,{L}_{B}^{* }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb5f9ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> with a correction for catalog incompleteness. Repeating the calculation for GW190814, we obtain <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{H}}}_{0}={67}_{-26.0}^{+41.0}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb5f9ieqn3.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup> and <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{H}}}_{0}={71}_{-30.0}^{+34.0}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb5f9ieqn4.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup> for <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{B}\geqslant 0.001\,{L}_{B}^{* }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb5f9ieqn5.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA ${L}_{B}\geqslant 0.626\,{L}_{B}^{* }$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb5f9ieqn6.gif" xlink:type="simple" /> </jats:inline-formula>, respectively. Combining the posteriors for both events yields <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{H}}}_{0}={70}_{-18.0}^{+29.0}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb5f9ieqn7.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup>, demonstrating the improvement on constraints when using multiple gravitational-wave events. We also confirm the results of other works that adopt this method, showing that increasing the <jats:italic>L</jats:italic> <jats:sub> <jats:italic>B</jats:italic> </jats:sub> threshold enhances the posterior structure and slightly shifts the distribution’s peak to higher H<jats:sub>0</jats:sub> values.We repeat the joint inference using the low-spin PhenomPNRT and the newly available combined (SEOBNRv4PHM + IMRPhenomPv3HM) posterior samples for GW170817 and GW190814, respectively, achieving a tighter constraint of <jats:inline-formula> <jats:tex-math> <?CDATA ${{\rm{H}}}_{0}={69}_{-14.0}^{+29.0}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb5f9ieqn8.gif" xlink:type="simple" /> </jats:inline-formula> km s<jats:sup>−1</jats:sup> Mpc<jats:sup>−1</jats:sup>.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>When observing spectral lines in the optically thin corona, line-of-sight (LOS) effects can strongly affect the interpretation of the data, especially in regions just above the limb. We present a semiempirical forward model, called the Global Heliospheric Optically thin Spectral Transport Simulation (GHOSTS), to characterize these effects. GHOSTS uses inputs from several other models to compute nonequilibrium ionization states (which include the solar-wind freezing-in effect) for many ions. These are used to generate ensembles of simulated spectral lines that are examined in detail, with emphasis on (1) relationships between quantities derived from observables and the radial variation of the observed quantities, (2) the behavior of thermal and nonthermal components of the line width, and (3) relative contributions of collisionally excited and radiatively scattered photons. We find that rapidly changing temperatures in the low corona can cause ion populations to vary dramatically with height. This can lead to line width measurements that are constant with height (a “plateau” effect) even when the temperature is increasing rapidly, as the plane of sky becomes evacuated and the foreground/background plasma dominates the observation. We find that LOS effects often drive the velocity width to be close to the plane-of-sky value of the wind speed, despite it flowing perpendicularly to the LOS there. The plateau effect can also cause the nonthermal component of the line width to greatly exceed the solar wind velocity at the observation height. Lastly, we study how much of the LOS is significant to the observation, and the importance of including continuum in the solar spectrum when computing the radiatively scattered emission.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>After 26 yr in outburst, the black hole X-ray binary GRS 1915+105 dimmed considerably in early 2018; its flux dropped sharply in mid-2019, and it has remained faint ever since. This faint period, the “obscured state,” is punctuated by occasional X-ray flares, many of which have been observed by NICER as part of our regular monitoring program. Here we present detailed time-resolved spectroscopy of one bright flare, whose spectrum shows evidence of high column density partial covering absorption and extremely deep absorption lines (equivalent widths over 100 eV in some cases). We study the time-dependent ionization of the obscuring gas with <jats:sc>xstar</jats:sc>, ultimately attributing the absorption to a radially stratified absorber of density ∼10<jats:sup>12</jats:sup>–10<jats:sup>13</jats:sup> cm<jats:sup>−3</jats:sup> at ∼few ×10<jats:sup>11</jats:sup> cm from the black hole. We argue that a vertically extended outer disk could explain this obscuration. We discuss several scenarios to explain the obscured state, including massive outflows, an increase in the mass accretion rate, and changes in the outer disk that herald the approach of quiescence, but none are entirely satisfactory. Alternative explanations, such as obscuration by the accretion stream impact point, may be testable with current or future data.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present a measurement of baryonic acoustic oscillations (BAOs) from Ly<jats:italic>α</jats:italic> absorption and quasars at an effective redshift <jats:inline-formula> <jats:tex-math> <?CDATA $z=2.33$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb085ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> using the complete extended Baryonic Oscillation Spectroscopic Survey (eBOSS). The 16th and final eBOSS data release (SDSS DR16) contains all data from eBOSS and its predecessor, the Baryonic Oscillation Spectroscopic Survey (BOSS), providing 210,005 quasars with <jats:italic>z</jats:italic> <jats:sub> <jats:italic>q</jats:italic> </jats:sub> &gt; 2.10 that are used to measure Ly<jats:italic>α</jats:italic> absorption. We measure the BAO scale both in the autocorrelation of Ly<jats:italic>α</jats:italic> absorption and in its cross-correlation with 341,468 quasars with redshift <jats:italic>z</jats:italic> <jats:sub> <jats:italic>q</jats:italic> </jats:sub> &gt; 1.77. Apart from the statistical gain from new quasars and deeper observations, the main improvements over previous work come from more accurate modeling of physical and instrumental correlations and the use of new sets of mock data. Combining the BAO measurement from the auto- and cross-correlation yields the constraints of the two ratios <jats:inline-formula> <jats:tex-math> <?CDATA ${D}_{H}(z\,=2.33)/{r}_{d}=8.99\pm 0.19$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb085ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA ${D}_{M}(z=2.33)/{r}_{d}=37.5\pm 1.1$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb085ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, where the error bars are statistical. These results are within 1.5<jats:italic>σ</jats:italic> of the prediction of the flat-ΛCDM cosmology of Planck (2016). The analysis code, <jats:monospace>picca</jats:monospace>, the catalog of the flux transmission field measurements, and the Δ<jats:italic>χ</jats:italic> <jats:sup>2</jats:sup> surfaces are publicly available.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We explore constraints on the equation of state (EOS) of neutron-rich matter based on microscopic calculations up to nuclear densities and observations of neutron stars. In a previous work we showed that predictions based on modern nuclear interactions derived within chiral effective field theory and the observation of two-solar-mass neutron stars result in a robust uncertainty range for neutron star radii and the EOS over a wide range of densities. In this work we extend this study, employing both the piecewise polytrope extension from Hebeler et al. as well as the speed of sound model of Greif et al., and show that moment of inertia measurements of neutron stars can significantly improve the constraints on the EOS and neutron star radii.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>It is difficult to discover pulsars via their gamma-ray emission because current instruments typically detect fewer than one photon per million rotations. This creates a significant computing challenge for isolated pulsars, where the typical parameter search space spans wide ranges in four dimensions. It is even more demanding when the pulsar is in a binary system, where the orbital motion introduces several additional unknown parameters. Building on earlier work by Pletsch &amp; Clark, we present optimal methods for such searches. These can also incorporate external constraints on the parameter space to be searched, for example, from optical observations of a presumed binary companion. The solution has two parts. The first is the construction of optimal search grids in parameter space via a parameter space metric, for initial semicoherent searches and subsequent fully coherent follow-ups. The second is a method to demodulate and detect the periodic pulsations. These methods have different sensitivity properties than traditional radio searches for binary pulsars and might unveil new populations of pulsars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>PSR J1809–1917 is a young (<jats:italic>τ</jats:italic> = 51 kyr) and energetic (<jats:inline-formula> <jats:tex-math> <?CDATA $\dot{E}=1.8\times {10}^{36}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabaf4bieqn1.gif" xlink:type="simple" /> </jats:inline-formula> erg s<jats:sup>−1</jats:sup>) radio pulsar powering an X-ray pulsar wind nebula (PWN) that exhibits morphological variability. We report on the results of a new monitoring campaign by the Chandra X-ray Observatory (Chandra), carried out across six epochs with a ∼7 week cadence. The compact nebula can be interpreted as a jet-dominated outflow along the pulsar’s spin axis. Its variability can be the result of Doppler boosting in the kinked jet, whose shape changes with time (akin to the Vela pulsar jet). The deep X-ray image, composed of 405 ks of new and 131 ks of archival Chandra data, reveals an arcminute-scale extended nebula (EN) whose axis of symmetry aligns with both the axis of the compact nebula and the direction toward the peak of the nearby TeV source HESS J1809–193. The EN’s morphology and extent suggest that the pulsar is likely moving through the ambient medium at a transonic velocity. We also resolved a faint 7′ long nonthermal collimated structure protruding from the PWN. It is possibly another instance of a “misaligned outflow” (also known as a “kinetic jet”) produced by high-energy particles escaping the PWN’s confinement and tracing the interstellar magnetic field lines. Finally, taking advantage of the 536 ks exposure, we analyzed the point sources in the J1809 field and classified them using multiwavelength data. None of the classified sources in the field can reasonably be expected to produce the extended TeV flux in the region, suggesting that PSR J1809–1917 is indeed the counterpart to HESS/eHWC J1809–193.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Accurate active galactic nucleus (AGN) identifications and spatially resolved host galaxy properties are a powerful combination for studies of the role of AGNs and AGN feedback in the coevolution of galaxies and their central supermassive black holes. Here, we present robust identifications of 406 AGNs in the first 6261 galaxies observed by the integral field spectroscopy survey Mapping Nearby Galaxies at Apache Point Observatory. Instead of using optical line flux ratios, which can be difficult to interpret in light of the effects of shocks and metallicity, we identify the AGNs via mid-infrared Wide-field Infrared Survey Explorer colors, Swift/BAT ultrahard X-ray detections, NVSS and FIRST radio observations, and broad emission lines in SDSS spectra. We subdivide the AGNs into radio-quiet and radio-mode AGNs, and examine the correlations of the AGN classes with host galaxy star formation rates and stellar populations. When compared to the radio-quiet AGN host galaxies, we find that the radio-mode AGN host galaxies are preferentially elliptical, lie further beneath the star-forming main sequence (with lower star formation rates at fixed galaxy mass), have older stellar populations, and have more negative stellar age gradients with galactocentric distance (indicating inside-out quenching of star formation). These results establish a connection between radio-mode AGNs and the suppression of star formation.</jats:p>